You define your ideal endstate first and foremost, they define theirs, and you work towards a compromise.

You do not under any circumstances begin with a compromise and work towards a worse compromise.

You shift the Overton Window -

... describes a window of majority acceptable ideas in public discourse, from a spectrum of all possible options on an issue. It provides a plan of action to make the options one would prefer acceptable to the public by priming them not with the leaders of the political movement, but with media spokespeople.

The steps an idea takes to full legitimacy are roughly as follows:

Unthinkable
Radical
Acceptable
Sensible
Popular
Policy

- in your own favor, while resisting its shifting in the opposite direction.

Another clearly expressed article details it nicely:

An Introduction to the Overton Window of Political Possibilities

... What does a think tank do? Does it educate? Advocate policy? Should a think tank focus on short-term or long-term goals?

Among Joe Overton’s many contributions, he was instrumental in defining the role of the Mackinac Center in particular and think tanks in general. He understood that, regardless of how persuasive the think tank, lawmakers are constrained by the political climate. Therefore, Overton concluded, to be truly successful, the Mackinac Center should not focus on direct policy advocacy, but instead should focus on educating lawmakers and the public in an attempt to change the political climate.

To answer the inevitable questions about the role of a think tank, Overton developed an explanation that others have since dubbed the "Overton Window of Political Possibilities." Though his theory has roots in complex public choice economics, it boils down quite easily.[1]

Imagine, if you will, a yardstick standing on end. On either end are the extreme policy actions for any political issue. Between the ends lie all gradations of policy from one extreme to the other. The yardstick represents the full political spectrum for a particular issue. The essence of the Overton window is that only a portion of this policy spectrum is within the realm of the politically possible at any time. Regardless of how vigorously a think tank or other group may campaign, only policy initiatives within this window of the politically possible will meet with success. Why is this?

Politicians are constrained by ideas, even if they have no interest in them personally. What they can accomplish, the legislation they can sponsor and support while still achieving political success (i.e. winning reelection or leaving the party strong for their successor), is framed by the set of ideas held by their constituents — the way people think. Politicians have the flexibility to make up their own minds, but negative consequences await the elected officeholder who strays too far. A politician’s success or failure stems from how well they understand and amplify the ideas and ideals held by those who elected them.

In addition to being dependent on the ideas that form the boundaries of the political climate, politicians are also known to be self-interested and desirous of obtaining the best political result for themselves.[2] Therefore, they will almost always constrain themselves to taking actions within the "window" of ideas approved of by the electorate. Actions outside of this window, while theoretically possible, and maybe more optimal in terms of sound policy, are politically unsuccessful. Even if a few legislators were willing to stick out their necks for an action outside the window, most would not risk the disfavor of their constituents. They may seek the good of those who elected them, and even the good of the state or nation as a whole, but in pursuing the course they think is best, most will certainly take into account their political future. This is the heart of the Overton window theory.

So, if a think tank’s research and the principles of sound policy suggest a particular idea that lies outside the Overton window, what is to be done? Shift the window. Since commonly held ideas, attitudes and presumptions frame what is politically possible and create the "window," a change in the opinions held by politicians and the people in general will shift it. Move the window of what is politically possible and those policies previously impractical can become the next great popular and legislative rage.

Likewise, policies that were once acceptable become politically infeasible as the window shifts away from them. Think tanks can shape public opinion and shift the Overton window by educating legislators and the public about sound policy, by creating a vision for how things could be done, by conducting research and presenting facts, and by involving people in the exchange of ideas...

In addition to setting higher/better/larger goals, successful negotiators also don't bend from their publically-stated objectives until very late in the negotiationg process. They were teaching that to us twenty years ago in marketing seminars I attended for a Fortune 10 company. Clearly the Democratic leadership has been snoozing for the last twenty years if they weren't aware of the most elemental tools available for negotiating skills development. I suspect that the Republicans will be experiencing a nasty shock when increasingly more Democrats begin applying modern techniques toward dealing with Republican tactics.